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Technicians at a paper mill were puzzled by hydraulic hoses pulling loose from their end fittings. Before this problem began, the workers had replaced a screw-in safety relief valve with an incorrect valve—its P and T ports were reversed.
This relief valve caused the swashplate in the hydraulic system’s pressure-compensated piston pumps to break a couple of times within a two-month period. The cause was determined to be pressure spikes that reached 9,000 psi, even though the pump’s compensator setting was 1,000 psi. These spikes resulted from a safety relief valve not functioning properly. The relief valve problem was discovered and corrected, but then the hose-end failures began.
Technicians made hose assemblies for various machines of the paper line in a room equipped with the proper fittings for the hose they stocked. Workers made sure the hose was inserted into the fitting at the proper depth and that they used the proper die set for crimping the fittings onto the hose. The working pressure rating for each hose assembly was 400 to 500 psi higher than system pressure.
As the hose failures occurred, technicians looked, again, for pressure spikes, but couldn’t detect any.
What was causing the hoses to pull away from their end fittings?
Find the Solution
Think you know the answer to this month’s problem? Submit your solution by e-mailing Mindy Timmer. All correct solutions submitted by June 2, 2016, will be entered into a random drawing for a $50 gift card. The winner will be notified, and his or her name will be printed in a future issue. Only one gift card will be awarded to any participant within a calendar year.
Congratulations to John Leyzorek, of Leyzorek Machine & Tool Co., Marlinton, W.V., who won our April puzzler by having his name picked from those who correctly answered that problem. A $50 gift card is in the mail to him.
Solution to Last Month’s Challenge: HPU Is Too Noisy
Local atmospheric pressure should be a consideration when shipping a hydraulic power unit (HPU) to a destination at a high elevation. For example, Denver lies more than 5,000 ft. above sea level, and its normal atmospheric pressure is about 12.1 psia. This is in contrast to the 14.7 psia often considered at sea level. This 2.60-psia difference doesn’t seem like much, but it means less pressure is available to force oil up the pump’s intake line. As a result, the pump must create a stronger vacuum to compensate, causing the cavitation noise to increase.
We had to remove the pump-motor unit and remount it to the lower side of the reservoir to provide a flooded intake. This now added the weight of the oil to the atmospheric pressure pushing the oil into the pump. Consequently, the HPU’s noise decreased to only 82 dBA while operating in Cincinnati.
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This file type includes high resolution graphics and schematics when applicable.
